The use of electrical measurements in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications is well known. Such techniques may be utilized to determine a subterranean formation resistivity, which, along with formation porosity measurements, is often used to indicate the presence of hydrocarbons in the formation. For example, it is known in the art that porous formations having high electrical resistivity often contain hydrocarbons, such as crude oil, while porous formations having low electrical resistivity are often water saturated. It will be appreciated that the terms resistivity and conductivity are often used interchangeably in the art. Those of ordinary skill in the art will readily recognize that these quantities are reciprocals and that one may be converted to the other via simple mathematical calculations. Mention of one or the other herein is for convenience of description, and is not intended in a limiting sense.
Directional resistivity measurements are also commonly utilized to provide information about remote geological features (e.g., remote beds, bed boundaries, and/or fluid contacts) not intercepted by the measurement tool. Such information includes, for example, the distance from and direction to the remote feature. In geosteering applications, directional resistivity measurements may be utilized in making steering decisions for subsequent drilling of the borehole. For example, an essentially horizontal section of a borehole may be routed through a thin oil bearing layer. Due to the dips and faults that may occur in the various layers that make up the strata, the distance between a bed boundary and the drill bit may be subject to change during drilling. Real-time distance and direction measurements may enable the operator to adjust the drilling course so as to maintain the bit at some predetermined distance from the boundary layer. Directional resistivity measurements also enable valuable geological information to be estimated, for example, including the dip and strike angles of the boundary as well as the vertical and horizontal conductivities of the formation.
Methods are known in the art for making LWD directional resistivity measurements. For example, LWD directional resistivity tools commonly measure or estimate a cross-component (e.g., the Hzx component) of an electromagnetic wave as the tool rotates in the borehole (e.g., during drilling). Various tool configurations are known in the art for measuring such cross-components. For example, U.S. Pat. No. 6,181,138 to Hagiwara teaches a method that employs an axial transmitting antenna and three co-located, circumferentially offset tilted receiving antennae. U.S. Pat. No. 6,969,994 to Minerbo et al., U.S. Pat. No. 7,202,670 to Omeragic et al., and U.S. Pat. No. 7,382,135 to Li et al teach a method that employs an axial transmitting antenna and two axially spaced tilted receiving antennae. The receiving antennae are further circumferentially offset from one another by an angle of 180 degrees. U.S. Pat. Nos. 6,476,609, 6,911,824, 7,019,528, 7,138,803, and 7,265,552 to Bittar teach a method that employs an axial transmitting antenna and two axially spaced tilted receiving antennae in which the tilted antennae are tilted in the same direction. While tilted antennae have been utilized commercially, one drawback with their use is that they transmit and/or receive mixed mode electromagnetic waves which do not allow small (e.g., transversal) signals to be easily separated out in the presence of measurement noises.
U.S. Pat. Nos. 7,057,392 and 7,414,407 to Wang et al teach a method that employs an axial transmitting antenna and two longitudinally spaced transverse receiving antennae. When the transmitter is fired each receiver measures the Hzx cross-component. These cross-components are then averaged (combined additively) to suppress tool bending effects. In order to make reliable measurements, the transmitter and/or receiver gain must remain constant, which can be problematic as the borehole temperature and pressure commonly fluctuate in downhole operations. Moreover, variations in the transmitter and/or receiver gain from tool to tool can erode the accuracy and consistency of the measurements.
While the above described methods (and the associated LWD resistivity tools) have been used commercially, there remains a need for further improved methods for making LWD directional resistivity measurements.